After two years of delays and hundreds of millions of dollars of cost overruns, the most crucial component of the International Space Station finally made it into space Wednesday.

Now, Russian and US space officials face a do-or-die challenge: linking the module - which holds living quarters and flight-control systems - with the station.

While extraterrestrial construction has become more routine in recent years, this process is particularly tricky, with scientists largely turning the job over to an automated computer docking program. Moreover, with the module years behind schedule - costing the US an additional $3 billion - failure would be an embarrassment for NASA and a possibly fatal blow for Russia's once-proud space program.

Cash shortages and two failures of a usually reliable type of Russian rocket repeatedly delayed the launch of the component, called Zvezda. Taken together with other problems, that has added tension to the project and raised doubts in NASA and Congress about Russia's reliability.

The Russian piece of the space station already in orbit, for example, was built entirely with US money because of Russian shortfalls. It also had so many defects that NASA had to send up an extra shuttle flight to make repairs.

Zvezda has similar issues, warns Congress's General Accounting Office. Among other things, both modules lack adequate space-debris and meteor protection, it says. NASA officials have assured Congress that they're working with the Russians to fix these problems in orbit.

The US investment in the International Space Station (ISS) is immense. It is taking up $2.3 billion of NASA's $14 billion annual budget. If the docking procedure is successful, the first crew will inhabit the nascent station later this year.

The galactic tinkertoy will be put together over the next two weeks. During that time, Russian controllers will position Zvezda to rendezvous with the station. From that point, the automatic docking system, which the Russians have used on Mir, will take over.

The system will gather data such as Zvezda's approach velocity, then guide the pieces of the ISS that were already in orbit through complex maneuvers that include 180-degree rotations. Much of this occurs out of range of Russian ground stations.

If all goes well, the two will be placed within a few hundred feet of each other when ground control is able to communicate with them and guide final docking.

If the first attempt fails, the system can make a second try. If that fails, specially trained cosmonauts - the "zero crew" - are standing by. They will fly to Zvezda and complete the docking manually.

Once assembled, the components will form a 116-foot-long complex that will be a true space station that can be permanently inhabited. Russia is readying a freighter to bring 4,000 pounds of supplies to the complex. NASA is preparing the shuttle Atlantis for a mission to unload those supplies and check out the station. If these and further planned missions are successful, a permanent crew should reach the station by year's end.

This would start a new phase of the 16-nation space station program, with components being added over the next few years. NASA plans as many as nine shuttle missions to the station in the next year.

Grateful that Russia took a key step with the launch Wednesday, NASA administrator Daniel Goldin said, "The Russians have gone through all sorts of difficulty with their economy, the political changes, and a whole variety of problems, and they came through and did what they said they were going to do."

Yet the US-Russian partnership still has problems to solve. NASA is wary of Russia's attempt to keep Mir operating using private sponsors. The ISS depends on regular service by Russian Progress vehicles as well as NASA shuttles. It's not clear that Russia will be able to service both the station and Mir.